Electrical grids are used to distribute electricity from usually a number of energy generators in large areas to many users and to supply households and industry with energy. Energy generators, usually in the form of power plants, provide the energy required for this. In general, the generation of electricity is planned and provided with regard to the forecast consumption.
However, unplanned fluctuations can occur both during generation and during consumption of energy. These may arise on the energy generator side for example as a result of a power plant or part of the electrical grid failing or, for example in the case of renewable energy sources such as wind, the energy generation being higher or lower than forecast. It is also possible with respect to the consumers for unexpectedly high or low levels of consumption to occur. The failure of part of the electrical grid, for example, cutting off some consumers from the energy supply, may lead to a sudden reduction in the electricity consumption.
This generally leads to fluctuations in the grid frequency in electrical grids due to unplanned deviations of power generation and/or consumption. In Europe, for example, the desired AC frequency is 50.00 Hz. A reduction in consumption as compared with the planned level leads to an increase in the frequency when power is fed in as planned by the energy generators; the same applies to an increase in the electricity production as compared with the planned level when consumption is as planned. On the other hand, a reduction in the power produced by the energy generators as compared with the planned level leads to a reduction in the grid frequency when consumption is as planned; the same applies to an increase in consumption as compared with the planned level when generation is as planned.
For reasons of grid stability, it is necessary to keep these deviations within defined boundaries. For this purpose, depending on the degree and direction of the deviation, positive control power must be specifically provided by connecting additional generators or disconnecting consuming entities or negative control power must be specifically provided by disconnecting generators or connecting consuming entities. There is a general need for cost-effective and efficient provision of these supplies of control power, where the requirements for the capacities to be maintained and the dynamic characteristic of the control power sources or sinks can vary according to the characteristics of the electrical grid.
In Europe, for example, there is a code of practice (UCTE Handbook), which describes three different categories of control power. In it, the respective requirements and the types of control power are also defined. The types of control power differ, inter alia, in the requirements in respect of the dynamic characteristic and the duration of power provision. Moreover, they are used differently with regard to the boundary conditions. Primary control power (PCP) is to be provided Europe-wide by all of the sources involved independently of the place of origin of the disturbance, this being substantially in proportion to the frequency deviation at the given time. The absolute maximum power has to be provided when there are frequency deviations of minus 200 mHz and below (in absolute terms), the absolute minimum power has to be provided when there are frequency deviations of plus 200 mHz and above. With regard to the dynamic characteristic, it holds true that, from the non-operative state, the respective maximum power (in terms of the absolute value) must be provided within 30 seconds. By contrast, secondary control power (SCP) and minutes reserve power (MRP) are to be provided in the balancing spaces in which the disturbance has occurred. Their task is to compensate as quickly as possible for the disturbance and thus ensure that the frequency is restored as quickly as possible to the desired range, preferably at the latest after 15 minutes. With regard to the dynamic characteristic, less stringent requirements are made of the SCP and the MRP (5 and 15 minutes, respectively, until full power provision after activation); at the same time these powers should also be provided over longer periods of time than primary control power.
In the electrical grids operated heretofore, a large part of the control power is provided by conventional power plants, in particular coal and nuclear power plants. This results in two fundamental problems. Firstly, the conventional power plants providing control power are not operated at full load, and consequently at maximum levels of efficiency, but slightly below, in order to be able when required to provide positive control power, possibly over a theoretically unlimited time period.
For long-term provision of control power, therefore, the required control power sources generally have to be operated at partial load in order to be able to take up or output additional energy as necessary. If a power plant is used, for example, then this would have to be operated at partial load in order also to be able to provide additional positive control power as necessary. Analogously, a consumer would have to be operated at partial load in order to be able to increase the load in the event of additional negative control power being required.
These partial-load modes of operation are generally disadvantageous. In most conventional power plants (e.g. coal-fired power plants or gas-fired power plants) partial-load operation can result in a lower efficiency of the electricity generation and higher specific emissions. This holds true particularly if the load is very low, relative to the maximum power. Moreover, increased specific fixed costs arise when there is reduced utilization of capacity. In the case of consumers operated at partial load, productivity decreases, and so does the efficiency. An electrolysis installation used for chemical production has a lower productivity in accordance with the load reduction and only a smaller proportion of the consumed energy is converted into the product, that is to say that a larger amount of energy is required for the same amount of product.
Secondly, with increasing expansion and increasingly preferred use of renewable energy sources, there are fewer and fewer conventional power plants in operation, which however is often the basic prerequisite for providing supplies of control power.
For this reason, approaches have been developed for the increasing use of stores in order to store negative control power and to provide it as positive control power as necessary.
DE 10 2008 002 839 A1 discloses operating energy consumers in the form of elevators in such a way that unused elevators in an entire region are driven to upper stories in order to provide negative control power. In other words, if negative control power is required, the power of a consumer is increased.
DE 10 2009 018 126 A1 discloses a method for providing control power which involves generating and storing a flammable gas with renewable energy sources. The flammable gas can be converted back into electricity precisely in periods of time with high residual load of the electrical grid. In this case, therefore, the power of a gas-fired power plant is increased if a positive control power is required. What is disadvantageous here is that the gas-fired power plant is operated at high power and thus at high efficiency only when there is a full control requirement, that is to say only in rare cases.
What is disadvantageous here, therefore, is that currently there is no possibility of operating energy generators or energy consumers for providing control power as efficiently as possible just like during operation for providing power without control and thus with the best possible efficiency, and also over a relatively long time, in order to make available control power for stabilizing the electrical grid. Oversizing is uneconomic in any case.
The use of hydro pumped-storage plants for producing control power is prior art. In Europe, all three types of control power mentioned above are produced by pumped-storage facilities. Hydro pumped-storage plants are however also repeatedly cited as currently the most cost-effective technology for storing and retrieving preferably forms of renewable energy, to allow energy supply and demand to be better adapted to one another in terms of time. The potential for the expansion of storage capacities—in particular in Norway—is a controversial subject of discussion since use requires considerable capacities in power lines to be approved and installed. Consequently, use for energy-efficient load management is in competition with the provision of control power.
Against this background, in the area of primary control power many plans for also using other storage technologies, such as for example flywheel mass and battery stores, for the provision of control power have recently been investigated and described.
US 2006/122738 A1 discloses an energy management system comprising an energy generator and an energy store, wherein the energy store can be charged by the energy generator. This is intended to enable an energy generator that does not ensure uniform energy generation in normal operation, such as for example the increasingly favored renewable energy sources such as wind-power or photovoltaic power plants, to output their energy more uniformly into the electrical grid. A disadvantage of this is that, although a single power plant can be stabilized in this way, all other disturbances and fluctuations of the electrical grid cannot be counterbalanced, or can be counterbalanced only to a very limited extent.
It is known from WO 2010 042 190 A2 and JP 2008 178 215 A to use energy stores for providing positive and negative control power. If the grid frequency leaves a tolerance range around the wanted grid frequency, either energy is provided from the energy store or is taken up in the energy store in order to regulate the grid frequency. DE 10 2008 046 747 A1 also proposes operating an energy store in an island electrical grid in such a way that the energy store is used to compensate for consumption peaks and consumption minima. What is disadvantageous about this is that the energy stores do not have the necessary capacity to compensate for a relatively long disturbance or a plurality of disturbances one after another that act in the same direction with regard to the frequency deviation.
In the article “Optimizing a Battery Energy Storage System for Primary Frequency Control” by Oudalov et al., in IEEE Transactions on Power Systems, Vol. 22, No. 3, August 2007, the dependence of the capacity of an accumulator on technical and operational boundary conditions is determined in order that said accumulator can provide primary control power according to the European standards (UCTE Handbook). It has been found that, on account of storage and retrieval losses, in the long term repeated charging or discharging of the store at different time intervals is unavoidable. In this respect, the authors propose the periods of time in which the frequency is in the dead band (i.e. in the frequency range in which no control power is to be provided). Nevertheless, in the short term or temporarily the situation can occur that the store is overcharged. The authors propose for such cases the (limited) use of loss-generating resistors which in the extreme case take up the complete negative nominal control power, that is to say have to be designed for that. Besides the additional capital expenditure requirement for the resistors and the cooling thereof, this leads, however, as already mentioned by the authors themselves to more or less undesirable energy degradation, wherein the waste heat that arises generally cannot be utilized. The authors demonstrate that reduced usage of loss generation is possible only by means of a higher storage capacity, associated with higher capital expenditure costs.
Accumulators and other energy stores can take up or output energy very rapidly, as a result of which they are suitable, in principle, for providing PCP. What is disadvantageous about this, however, is that very large capacities of the accumulators have to be provided in order to be able to supply the control power also over a relatively long period of time or repeatedly. However, accumulators having a very high capacity are also very expensive.
On account of the losses during the storing and outputting of energy, the energy store, such as an accumulator for example, is discharged earlier or later in the event of statistically symmetrical deviation of the grid frequencies from the desired value as a result of operation. Therefore, it is necessary to charge the energy store more or less regularly in a targeted manner. This charging current may need to be paid for separately.
In the context of the invention it has been found that occasionally considerable quantities of energy are fed in or output monotonically as shown by an analysis of real frequency profiles by the inventors. This leads to a correspondingly high change in the state of charge for a given storage capacity. Large changes in the state of charge in turn tend to result in more rapid aging than small changes in the state of charge. Consequently, either the energy store reaches the end of its life sooner and has to be replaced sooner, or the capacity has to be increased a priori in order to reduce the relative change in the state of charge. Both result in an increase in the capital expenditure costs.
In addition, consistently complying with the guidelines for the prequalification of primary control technologies necessitates keeping corresponding power reserves available at any arbitrary time during operation and thus for any arbitrary state of charge of the energy store. This requirement (currently in Germany: the marketed primary control power for a duration of 15 min) has the effect that a corresponding capacity additionally has to be kept in reserve, this capacity increasing capital expenditure costs. In fact, such a reserve would (on a statistical basis) only be used very rarely.